It's probably a bug in the curl multi implementation:
In many many cases you keep the curl connection alive after a curl_exec().
With curl_multi that's not possible if you don't do this after every get_content() :
curl_multi_remove_handle($ch);
curl_multi_add_handle($ch);
I've not yet tested if this also breaks the keepalive connection but if you don't do this you'll just get the old previous response after each curl_multi_exec()